2.3 geophysical investigations - jaea · 2.3 geophysical investigations 2.3.1 aims ... geophysical...

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2.3 Geophysical Investigations

2.3.1 Aims

- To identify potential water-conducting features and impermeable argillaceous layers.

- To acquire information on the orientation and geometry of geological discontinuities.

- To acquire geophysical properties by continuous wire line logging to support the

geological and hydrochemical modelling.

- To characterise in situ neutron flux production for hydrochemical interpretations.

2.3.2 Work performed

Geophysical logging: A suite of borehole geophysical logging methods, consisting of

temperature, spontaneous potential, resistivity, micro-resistivity, natural gamma, gamma

spectrum, neutron, density, acoustic, X-Y calliper and deviation logging methods were

employed from the surface to the bottom of each borehole. In MSB-1, because the

groundwater level in the borehole was below 60 mabh, geophysical logging was conducted in

an injection state above the static water level.

BTV: BTV investigations were performed from the surface to the bottom of each borehole.

Below 92 mabh in MSB-3, the BTV investigation was performed through PQ rods installed to

the depth as casing pipe, because borehole collapse had occurred during drilling in the

interval from 91 to 102 mabh near the NNW fault.

2.3.3 Results

Geophysical logging:

- Continuous profiles of the geophysical properties were acquired from the surface to the

bottom of each borehole. Geophysical logs are presented in Figures 21, 22, 23 and 24.

- Conglomerate layers and coarse sandstone layers intercalated in the Akeyo Formation and

the Toki Lignite-bearing Formation and the unconformity between the Mizunami Group

and the Toki Granite are distinct anomalies on the spontaneous potential, resistivity, micro-

resistivity, neutron, acoustic and density logs. Although these coarse clastic layers in the

sedimentary formations are generally inferred to be permeable, the depths at which distinct

anomalies on the geophysical logs occurred are not always coincident with fluid loss zones

and anomalies identified by fluid logging (Figures 21, 22, 23 and 24, see section 2.4.3 for

relevant matters).

- Relatively high gamma anomalies (500 to 2,000 API above background) were identified in

the arkosic sandstone immediately above the basal conglomerate of the Toki-Lignite

bearing Formation in MSB-1, 2 and 3, and immediately above the unconformity between

the Mizunami Group and the Toki Granite in MSB-4.

- The tuffaceous sandstone and tuff layers above the top of the basal conglomerate and

below the mudstone or muddy sandstone in the Akeyo Formation were defined as low

resistivity sections in each borehole. According to the study in the Tono Mine area [6], the

sections are possibly impermeable layers in the sedimentary rocks.

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BTV:

- Distributions of fracture frequency and orientation in each borehole, presented in Figures

25 and 26, were observed in BTV images and also coincided with the data from the core

description.

- Subhorizontal fractures are dominant in the all sections of the sedimentary rocks and in the

granite, except for the granite in MSB-4, in which steep WNW striking fractures are

dominant. NS and NNW striking fractures dipping at steep angles towards the east are

dominant in the weathered granite in MSB-3.

- Two fracture zones, defined by an abrupt increase in cumulative fracture frequency, were

identified in the Akeyo Formation in MSB-1. In these zones, EW and NE striking fractures

dipping at steep to moderate angles to the south are predominant. Also, one fracture zone

was identified along the NNW fault in MSB-3, in which NW striking fractures dipping at

steep to moderate angles towards the east are predominant.

- Four water-conducting features identified in MSB-1, based on fluid loss during drilling, are

EW and EWE striking and dipping at steep to moderate angles towards the south or north.

2.3.4 Evaluations

- Some coarse clastic layers were identified as anomalies by the geophysical logging.

However the anomaly depths did not always coincide with the depths of fluid loss during

drilling nor did with the anomalies on the fluid logging.

- Although argillaceous layers were not identified, tuffaceous sandstone layers of low

resistivity were defined in part of the Akeyo Formation. They are possible to be

impermeable layers in the sedimentary rocks.

- Information was acquired on the orientation and geometry of many of the geological

discontinuities. But we could not acquire this data for the NNW fault, and some of the

fractures in the conglomerate and the weathered granite because of sludge accumulation on

the borehole wall and significant enlargement of borehole diameter, making it difficult to

get reliable BTV images.

- Continuous profiles of geophysical properties were acquired in each borehole to support

the geological and hydrochemical modelling.

- Relevant data on in situ neutron flux production was acquired in each borehole.

2.3.5 Lessons learned

- A unidirectional scan line on core should be drawn to accurately infer the orientation of

discontinuities, preparing for loss of BTV images because of sludge accumulation,

borehole enlargement and turbidity of drilling fluid.

- Logging methods applicable to extremely low groundwater level (unsaturated or partially

saturated conditions) should be included in working programme.

- Employment of other geophysical logging method should be considered for identification

of potential water-conducting features in sedimentary rocks, because depths of anomalies

from the borehole geophysics did not always coincide with the depths of fluid loss during

drilling and of the fluid logging anomalies.

Figure25FracturefrequencyandorientationinMSB-1and2

Frac

ture

zone

:abr

upti

ncre

ase

incu

mul

ativ

efra

ctur

efre

quen

cyco

inci

dew

ithhi

gher

fract

ure

occu

rrenc

eslo

gged

inco

re.

31

MSB

-1

Miz

unam

iGro

up

E-W

/05S

N84

E/58

S

WC

F2

WC

F1

N=5

5

WC

F3

WC

F4

Frac

ture

zone

1

Orie

ntat

ion

Toki

Gra

nite N79

W/2

7S

N40

E/10

E

N=1

0

Frac

ture

zone

2

0-3

0°30

-60°

60-9

0°Di

pAn

gle

BTV

Frac

ture

s

75.6

2

87.6

0

32.3

2

34.7

2

Frac

ture

zone

1

Frac

ture

zone

2

Frac

ture

Dens

ity(N

m-1

)Cu

mula

tive

Num

ber(

N)2

46

0 050

100

201.

00

Geo

logi

calO

bser

vatio

ns

DrillingDepth(mabh)

20 40 80 100

120

140

160

180

200

60

78.9

0

195.

3319

5.60

131.

32

179.

48

117.

10

69.7

8

2.00

8.52

LithostratigraphicalColumn

Tuffa

ceou

ssa

ndst

one

Tuffa

ceou

ssa

ndst

one

Arko

sic

sand

ston

e,M

udst

one,

Gran

ulecg

l.,Li

gnite

Basa

lco

nglom

erat

e

Basa

lco

nglom

erat

e

TokiLignite-bearingFormationAkeyoFormation

LithostratigraphicalDescriptions

Mud

ston

e

Mudd

ysan

dston

e

Silts

tone

No

core

Tuffa

ceou

ssa

ndst

one,

Tuff,

Gran

ulecg

l.

TokiGranite

Oidawara

F.

wea

ther

edzo

neme

dium-g

rained

biotite

granite

30.0

036

.00

0-3

0°30

-60°

60-9

0°Di

pAn

gle

Frac

ture

Dens

ity(N

m-1

)Cu

mula

tive

Num

ber(

N)2

46

0 050

100

Miz

unam

iGro

up

N=5

5

Orie

ntat

ion

Toki

Gra

nite

BTV

Frac

ture

s

00/0

0

N=1

9

N45

E/03

E

N=9

MSB

-2 DrillingDepth(mabh)

20 40 80 100

120

140

160

180

60

77.5

4

170.

9517

3.35

132.

10

69.3

0

49.0

0

38.5

0

180.

00


Geo

logi

calO

bser

vatio

ns

Tuffa

ceou

ssa

ndst

one

Arko

sic

sand

sotn

e,M

udst

one,

Gran

ulecg

l.,Li

gnite

Basal

conglo

merate

Basa

lco

nglom

erat

e



Mud

ston

e

Tuffa

ceou

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ndst

one,

Tuff

weath

ered

zone

medium

-grained

biotiteg

ranite

TokiGraniteN

oco

re7.

80

Figure26FracturefrequencyandorientationinMSB-3and4

32

Frac

ture

zone

:abr

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ncre

ase

incu

mul

ativ

efra

ctur

efre

quen

cyco

inci

dew

ithhi

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fract

ure

occu

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inco

re.

MSB


20 40 80 100

120

140

160

180

60

72.5

0

177.

70

190.

60

140.

90

88.7

0

54.1

0


Geo

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ns

3.80

42.2

2

199.

00


Tuffa

ceou

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one

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Gran

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l.,Li

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Basa

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Basa

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No

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ium

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faultzone

Miz

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N=3

8

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Gra

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N=3

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N33

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8EN33

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Frac

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zone

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W/6

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31E/

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ult

Wea

ther

edzo

ne

0-3

0°30

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0°Di

pAn

gle Frac

ture

zone

Frac

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Dens

ity(N

m-1

)Cu

mula

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Num

ber(

N)2

46

0 050

100

88.1

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103.

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BTV

Frac

ture

s

MSB


20 40 8060

76.6

0

93.9

0

63.4

0

Geo

logi

calO

bser

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ns

99.0

0


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l.,L

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TokiLignite-bearing

FormationAkeyoFormation


Mud

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e

No

core

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Tuff,

Gran

ulecg

l.

medium

-grained

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ranite

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34.4

530

.50

13.5

0

0-3

0°30

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0°Di

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Frac

ture

Dens

ity(N

m-1

)Cu

mula

tive

Num

ber(

N)2

46

0 050

100

BTV

Frac

ture

s

N70

W/8

4S

N74

W/6

4N

Miz

unam

iGro

up

N=2

4

Orie

ntat

ion

Toki

Gra

nite

N=2

N43

W/1

6W

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